Cavitation detection

ABSTRACT

There is disclosed an apparatus and method for detecting cavitation in fluid machines, for example pumps ( 100 ). In one embodiment a piezoelectric gasket ( 102 ) is used as a sensor to sense cavitation. In some embodiments highpass filters ( 302 ), ( 501 ) are used to detect ultrasonic acoustic signals in about the MHz range. If the energy in the MHz range is excessive then cavitation is deemed to be occurring and the speed of a motor ( 110 ) may be reduced in proportion to the degree of cavitation deemed to be occurring. In another embodiment (FIG.  5 ) the energy in the MHz range is normalised against the energy in the kHz range. Other sensors ( 600, 701 ) are also disclosed.

The present invention is concerned with the detection of cavitation influid mechanisms. In particular, the present invention is concerned withthe detection of cavitation in pumps for pumping a fluid (for example asupercritical fluid) or a liquid.

Cavitation is problematic phenomenon that can occur when a pump isoperated such that the pressure inside the pump drops below the vapourpressure of a liquid being pumped. Bubbles of the vapourised liquid areformed. When the bubbles collapse, damage can be caused to the pump.During severe cavitation the noise of collapsing bubbles may be audibleto a human. It would, however, be advantageous to detect the onset ofsevere and/or damaging cavitation so that the operating conditions ofthe pump can be modified (a minor amount of cavitation can be toleratedin some applications).

JP 11-037979 discloses a system for detecting cavitation in fluidmechanisms such as pumps. JP 11-037979 operates by comparing successiveacoustic waveform cycles from a pump. Each waveform cycle is decomposedinto a plurality of coefficients. The coefficients of successiveacoustic waveform cycles are compared on a term-by-term basis by takingthe dot product of successive acoustic waveforms. If successive acousticwaveform cycles are sufficiently similar then cavitation is deemed notto be occurring; if successive acoustic waveform cycles are sufficientlydifferent then cavitation is deemed to be occurring.

According to an aspect of the present invention, there is provided:

-   -   a transducer;    -   a highpass filter for highpass filtering a signal from the        transducer;    -   a reference receiver for receiving a threshold;    -   a comparator for comparing the highpass filtered signal with the        threshold.

According to other aspects of the invention, there are provided acousticsensors and a method of detecting cavitation.

An advantage of the present invention is that less signal processing isrequired compared to some prior art cavitation detection methods.Another advantage of some embodiments of the present invention is thatthey detect cavitation in the MHz range, which allows cavitation (or theonset of violent cavitation) to be more reliably identified.

In one embodiment of the present invention, the signal from the sensoris compared with a low pass filtered version of the signal from thesensor. An advantage of this embodiment is that it is less dependent onthe level of the signal from the sensor and thus this embodiment is moretolerant of uncertainties in the acoustic coupling of the sensor to apump being monitored.

DESCRIPTION OF FIGURES

FIG. 1 a shows a perspective view of a pump and a motor; FIG. 1 b showsa view of the pump and motor which enables a piezoelectric gasket to beseen.

FIG. 2 shows a view of the piezoelectric gasket of FIG. 1 b.

FIG. 3 shows a schematic illustration of a system which may be used todetect the onset of cavitation and reduce the speed of the motor ifcavitation is detected.

FIG. 4 shows an example of acoustic waveforms from a pump: (i) when thepump is operating without cavitation and (ii) during the onset ofcavitation.

FIG. 5 shows a schematic illustration of a system based on FIG. 3 butwhich compares differently filtered versions of the signal from asensor.

FIG. 6 shows a perspective view of a sensor that may be pressed againstthe housing of a pump in order to measure an acoustic signal from thepump.

FIG. 7 shows a cross-sectional view of a portion of a pump in which anacoustic sensor has been embedded.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 a and 1 b show a pump 100 coupled to an inlet pipe 101 by apiezoelectric gasket 102. A motor 110 has a shaft 110 that drives animpellor 112 of the pump 100.

FIG. 2 shows a more detailed view of the piezoelectric gasket 102. Inthis embodiment the piezoelectric gasket 102 is annular and has twoplanar faces. One of the faces is used to form a fluid seal against thepump 100; the other face is used to form a fluid seal against the inletpipe 101.

In this embodiment the piezoelectric gasket 102 comprises a polymerpiezoelectric material, for example PVDF (polyvinylidene fluoride),having a thickness of 50 μm. Leads 201 a, 201 b are connected torespective faces of the gasket 102. The piezoelectric gasket 102 may bemade from a single layer of film, a series of laminated layers or mayposses a structured or patterned electrode. When the piezoelectricgasket 102 is clamped between the pump 100 and the inlet pipe 101, thepiezoelectric gasket will convert acoustic signals from the pump intoelectrical signals. The piezoelectric gasket 102 can therefore be usedto detect acoustic signals from the pump 100.

In other embodiments an electrostatic shield may be provided around thegasket 102 and/or the leads 201 a, 201 b to reduce the influence ofexternal electrical signals.

In yet other embodiments, a ceramic piezoelectric material or a mixedpolymer-ceramic composite material may be used instead of a polymerpiezoelectric material. If a ceramic piezoelectric material is used thenit may be necessary to coat the ceramic with a polymer such as rubber toprevent cracking of the ceramic when the ceramic piezoelectric materialis clamped between the pump 100 and the inlet pipe 101.

An advantage of the use of a polymer piezoelectric material is that thethickness of the polymer may be more readily reduced than the thicknessof a ceramic piezoelectric material. A relatively thin material will ingeneral be better than a thicker material at detecting high frequencyacoustic signals; this is because a thin material is small compared tothe wavelength of a high frequency acoustic signal. For example, a 50 μmthick layer of PVDF has an upper frequency response of about 20 MHz andso cannot be effectively used to sense acoustic signals having afrequency of higher than about 20 MHz. At frequencies below 20 MHz thethickness of a thick 50 μm PVDF layer is significantly less than thewavelength of sound in, say, water. Also, a thinner material will have ahigher resonant frequency (the first resonance occurs when the thicknessof the material becomes equal to half of the acoustic wavelength), thusallowing higher frequency signals to be detected.

FIG. 3 shows a schematic illustration of a system which may be used todetect the onset of cavitation and reduce the speed of the motor ifcavitation is detected. FIG. 3 shows that the electrical signal from thepiezoelectric gasket 102 is amplified by an amplifier 301 and is thenfiltered by a highpass filter 302. (In alternative embodiments, it maybe preferable to filter and then amplify the signal from thepiezoelectric gasket 102.) In this embodiment the highpass filter 302attenuates signals having a frequency of less than 1 MHz. In thisembodiment the highpass filter 302 is a second order (i.e. attenuationincreases at 12 dB per octave) analogue filter although a digital filtercould be used in alternative embodiments.

The output of the highpass filter 302 is connected to a detector 303. Inthis embodiment the detector 303 is an envelope detector and convertsthe signal from the highpass filter 302 into a voltage indicative of thestrength of the signal from the highpass filter 302. In otherembodiments, the detector 303 may comprise an amplitude sensor or an RMS(root-mean-square) detector.

The output of the detector 303 is connected to a comparator 304. Thecomparator 304 compares the voltage from the detector 303 with areference signal received from a reference input 305. In this embodimentthe comparator 304 subtracts the reference signal from the detectedvoltage. In other embodiments, the comparator 304 may divide thedetected voltage by the reference signal to form a ratio.

In this embodiment, the output of the comparator 304 is connected to amotor controller 306 which controls the speed of the motor 110. In theevent that the comparator 304 indicates excessive cavitation, the motorcontroller 305 reduces the speed of the motor 110 in order to reducecavitation. In other embodiments, other methods may be used to reducecavitation. For example, the head of pressure at the inlet to the pump100 may be increased.

FIG. 4 shows an example of acoustic waveforms from the pump 100, andshows the amplitude of the acoustic waveform versus frequency. Curve 401(the solid line) shows the amplitude of the acoustic waveform versusfrequency when the pump is operating without significant cavitation.Curve 402 (the dotted line) shows the amplitude of the acoustic waveformversus frequency and during the onset of violent cavitation.

As can be seen from FIG. 4, during cavitation the amplitude of theacoustic waveform increases slightly at kHz frequencies butsignificantly at MHz frequencies. Prior art cavitation detectorsgenerally attempt to detect cavitation having a frequency of 100 s ofkHz. Embodiments of the present invention may detect cavitation having afrequency at least 0.5 MHz, at least 1 MHz, at least 2 MHz, at least 4MHz or at least 8 MHz, by altering the cutoff frequency of highpassfilter 302 as appropriate.

FIG. 5 shows a schematic illustration of a system based on FIG. 3 butwhich compares differently filtered versions of the signal from thepiezoelectric gasket 102. FIG. 5 shows that the output from theamplifier 301 is filtered by a bandpass filter 501 (instead of thehighpass filter 302). In this embodiment the bandpass filter 50lattenuates frequencies that lie outside the passband of 1 MHz to 5 MHz.The output from the amplifier 301 is also filtered by a lowpass filter502, detected by a detector 503 and used as the reference input to thecomparator 304. In this embodiment the lowpass filter 502 has a cutofffrequency of 1 kHz and thus attenuates frequencies higher than 1 kHz. Inalternative embodiments, the cutoff frequency of the lowpass filter 502may instead be at most 10 kHz, 100 kHz or 1 MHz.

As those skilled in the art will appreciate, the bandpass filter 501 canconceptually be regarded as a 1 MHz highpass filter in series with a 5MHz lowpass filter (even though the bandpass filter 501 may actually beimplemented as a single bandpass filter rather than as concatenatedhighpass and lowpass filters). Thus FIG. 5 includes a highpass filterfunction that is equivalent to the highpass filter 302 of FIG. 3. Insome embodiments the upper frequency response of the piezoelectricgasket 102 may be used to define the lowpass filter of the bandpassfilter 501.

The system of FIG. 5 compares the acoustic energy emanating from thepump 100 in the frequency range of 1 MHz to 5 MHz with the acousticenergy below 1 kHz. If there is an excess of energy in the range 1 MHzto 5 MHz then the pump 100 is deemed to be undergoing excessivecavitation and the motor controller 305 is used to reduce the speed ofthe motor 110 (and thus of the pump 100) accordingly.

An advantage of the system of FIG. 5 compared to the system of FIG. 3 isthat FIG. 5 is more tolerant of imperfect acoustic coupling between thepiezoelectric gasket 102 and the pump 100, and is also more tolerant ofimperfect coupling due to the distance that the ultrasonic sound has totravel from the cavitating surface inside the pump 100 to the casing ofthe pump 100. The system of FIG. 3 cannot account for poor acousticcoupling (poor acoustic coupling could erroneously result in anindication that cavitation is not occurring). In contrast, the system ofFIG. 5 normalises the energy in the frequency band 1 MHz to 5 MHzagainst the energy in the frequency range below 1 kHz. Thus if thepiezoelectric gasket 102 is not well coupled to the pump 100, the signalin the frequency band 1 MHz to 5 MHz and the signal in the frequencyrange below 1 kHz will both be reduced. Thus the system of FIG. 5 can atleast partially compensate for imperfect acoustic coupling.

FIG. 6 shows a perspective view of a sensor 600 that may be pressedagainst the housing of a pump in order to measure an acoustic signalfrom the pump. The sensor 600 may be used during the commissioning of apump to determine operating conditions for the pump that do not involveexcessive cavitation. Once the operating conditions have beendetermined, the sensor 600 may be more easily removed from the pumpthan, say, the piezoelectric gasket 102. On the other hand, an advantageof the piezoelectric gasket 102 is that it allows continuous real-timemonitoring of the pump 100.

As shown, the sensor 600 comprises an acoustic coupler 601 which may bepressed against the housing of a pump. In this embodiment the acousticcoupler 601 is formed of rubber and couples acoustic energy to threePVDF layers 602 a, 602 b, 602 c. In this embodiment each of the threePVDF layers 602 is substantially planar. The use of three PVDF layersimproves the output voltage of the sensor 600 by a factor of about threebut also reduces the upper frequency limit of the sensor 600 (comparedto using a single PVDF layer; the reduction in the upper frequency limitis due to the increased thickness of the three PVDF layers compared to asingle layer). In this embodiment the Shore hardness of the elastomericcoupler 601 is preferably in the range 10 to 20 although in otherembodiments the Shore hardness may be less than 10 or more than 20.

The sensor 600 also has, in this embodiment, a metal shank 603 whichacts as a grip portion to allow a user to hold the acoustic coupler 601of the sensor 600 against the exterior of a pump or against pipesfastened to the pump. A lead 604 is used to connect the sensor 600 tocircuitry (not shown),

In alternative embodiments an acoustic sensor (not shown) may befastened or bonded (for example using an adhesive) against the exteriorof a pump. In such embodiments the acoustic coupler 601 and/or the metalshank 603 may not be required.

FIG. 7 shows a cross-sectional view of a portion of a pump 700 (themajority of the pump 700 is shown in phantom lines) in which an acousticsensor 701 has been embedded. The acoustic sensor 701 is embedded in arecess 702 in the casing 703 of the pump 700. An advantage of thisembodiment is that the acoustic sensor 701 is close to the fluid inwhich cavitation could occur. In contrast, the embodiment of FIG. 1requires the acoustic signal to travel from the interior of the pump 100to the piezoelectric gasket 102.

As those skilled in the art will appreciate, the present invention maybe used to reduce cavitation in pumps, for example centrifugal pumps oraxial pumps, or in other fluid mechanisms.

In some situations it may be advantageous to reduce the influence ofelectrical noise. The electrical signal from an acoustic sensortypically requires significant amplification and thus there is the riskof inadvertently picking up stray electrical signals. Although theelectrostatic shield discussed above in connection with FIG. 2 may besufficient, in other embodiments a calibration step may be performed. Insuch embodiments, the acoustic signal is measured under conditions underwhich it is known that no cavitation is occurring (for example when themotor 110 is stationary). If a signal, for example having a frequency of2 MHz, is detected under the no-cavitation condition then anelectrically configurable notch filter may be used to suppress theextraneous 2 MHz signal in order to avoid false alarms of cavitationwhen the motor 110 is running.

In some embodiments, for example when the acoustic sensor is to befitted to plant machinery which is in use and cannot be stopped, it maybe inconvenient to stop the motor(s) that drive pump(s). In such cases,it may be more convenient to detach the acoustic sensor from the body ofa pump, thereby providing a “normalisation” signal in which cavitationis not occurring. Alternatively, the acoustic sensor may be allowed todangle in air; the sensor may still pick up background electricalsignals (particularly if the pump is in an electrically noisyenvironment) and the background electrical noise may thus be used as anormalisation signal.

In some embodiments, if the signal from the piezoelectric gasket 102 issufficiently strong then it may not be necessary to use the amplifier301.

There is disclosed an apparatus and method for detecting cavitation influid machines; for example pumps 100. In one embodiment a piezoelectricgasket 102 is used as a sensor to sense cavitation. In some embodimentshighpass filters 302, 501 are used to detect ultrasonic acoustic signalsin about the MHz range. If the energy in the MHz range is excessive thencavitation is deemed to be occurring and the speed of a motor 110 may bereduced in proportion to the degree of cavitation deemed to beoccurring. In another embodiment (FIG. 5) the energy in the MHz range isnormalised against the energy in the kHz range. Other sensors 600, 701are also disclosed.

The disclosures in the abstract of the present application, and theentirety of GB 0714695.4 (from which the present application claimspriority), are hereby incorporated by reference.

1. A system for detecting and controlling the occurrence of cavitationduring operation of a fluid mechanism, comprising: an acoustic sensor; ahighpass filter having a cut-off frequency greater than or equal to 1MHz for filtering a signal from the acoustic sensor; a first detectorfor providing a signal indicative of the energy in the highpass filteredsignal from the acoustic sensor; a reference receiver for receiving areference value signal; a comparator for comparing the signal from thefirst detector with the reference value signal and providing anindication if the signal from the first detector exceeds the referencevalue signal; and a controller operable to control an operatingcondition of a fluid mechanism to reduce cavitation in the event thatthe comparator indicates that the signal from the first detector exceedsthe reference value signal.
 2. A system according to claim 1, comprisingan amplifier to amplify the signal from the acoustic sensor. 3.(canceled)
 4. A system according to claim 1, wherein the highpass filterforms part of a bandpass filter comprising a lowpass filter.
 5. A systemaccording to claim 4, wherein the lowpass filter comprises an upperresponse frequency of the acoustic sensor.
 6. A system according toclaim 4, wherein the cutoff frequency of the lowpass filter section ofthe bandpass filter is 5 MHz or 20 MHz.
 7. A system according to claim1, comprising: a lowpass filter for filtering a signal from the acousticsensor; and a second detector for providing a signal indicative of theenergy in the lowpass filtered signal from the acoustic sensor, whereinthe reference receiver is arranged to receive the signal from the seconddetector as the reference value signal.
 8. A system according to claim7, wherein the lowpass filter has a cutoff frequency of at most one ofIkHz, 1OkHz, 10OkHz and IMHz.
 9. (canceled)
 10. A system according toclaim 1, wherein the controller comprises a motor controller and whereinthe motor controller is operable to reduce the speed of a motor in theevent that the signal from the first detector exceeds the referencevalue signal.
 11. A system according to claim 10, comprising a motor.12. A system according to claim 11, comprising a pump connected to themotor, wherein the acoustic sensor is located relative to the pump tosense cavitation occurring within the pump.
 13. A system according toclaim 1, wherein the acoustic sensor comprises a gasket comprising apiezoelectric material.
 14. A system according to claim 13, wherein thepiezoelectric material comprises a polymer piezoelectric material.
 15. Asystem according to claim 14, wherein the gasket comprises PVDF.
 16. Asystem according to claim 13, wherein the gasket is located at an inletto a pump.
 17. A system according to claim 1, wherein the acousticsensor comprises an acoustic rubber coupler, one or more polymerpiezoelectric layers and a grip portion.
 18. A system according to claim17, wherein the acoustic rubber coupler comprises rubber having a Shorehardness in the range 10 to
 20. 19. A system according to claim 1,wherein the acoustic sensor is provided in a recess in a casing of apump.
 20. An acoustic sensor comprising a gasket comprising a polymerpiezoelectric material.
 21. An acoustic sensor comprising an acousticcoupler, one or more polymer piezoelectric layers and a grip portion.22. A pump having an acoustic sensor provided in a recess in a casing ofthe pump.
 23. (canceled)
 24. (canceled)
 25. (canceled)
 26. A method ofdetecting and controlling the occurrence of cavitation in a fluidmachine, comprising the steps of: receiving a signal from an acousticsensor that is acoustically coupled to the fluid machine; high passfiltering the signal from the acoustic sensor by means of a high passfilter having a cut-off frequency greater than or equal to 1 MHz;providing a signal indicative of the energy in the high pass filteredsignal from the acoustic sensor; comparing the signal indicative of theenergy with a reference value signal and providing an indication if thesignal indicative of the energy exceeds the reference value signal; andcontrolling an operating condition of a fluid mechanism to reducecavitation in the event that the comparator indicates that the signalfrom the first detector exceeds the reference value signal.